Many types and kinds of valves have been developed over the years for wellhead pressure and flow control. Many companies manufacture and market this kind of equipment. Examples are: Seaboard Wellhead, Inc., Houston, Tex.; Cameron Iron Works, Inc., Houston, Tex.; FMC Corporation, Houston, Tex.; Sii Willis (Division of Smith International), Houston, Tex.; W-K-M (Division of Joy Manufacturing) Houston, Tex.; Thornhill Craver Company, (Division of Joy Manufacturing) Houston, Tex.; Gray Tool Company, Subsidiary of Combustion Engineering, Houston, Tex.; and others.